A daily dose of a widely used antibiotic can prevent some infections with syphilis, gonorrhea and chlamydia, potentially a new solution to the escalating crisis of sexually transmitted infections, scientists reported Thursday.

Study Was Small

Their study was small and must be confirmed by more research. Scientists still have to resolve significant questions, including whether STIs might become resistant to the antibiotic and what effect it could have on healthy gut bacteria in people taking it every day.

The approach would be recommended primarily to people at elevated risk of STIs during certain periods, said Dr. Jeffrey Klausner, an infectious diseases physician at the University of Southern California who was not involved in the new work.

“The number of people who are really going to be offered this and take this is still very small,” he said. “In general, the more choices we have for people, the more prevention options we have, the better.”

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Results Presented Next Week

The results will be presented next week at a conference of the International AIDS Society in Munich.

The United States now has the highest rate of new syphilis infections since 1950, the Centers for Disease Control and Prevention reported in January. In 2022, the last year for which data are available, there were 1.6 million cases of chlamydia and nearly 650,000 new cases of gonorrhea.

Previous studies have shown that the antibiotic doxycycline substantially cuts the risk of new infections if taken within 72 hours after unprotected sex. The CDC now recommends taking doxycycline after “oral, vaginal or anal sex.”

The recommendation applies only to gay and bisexual men and transgender women who have had an STI within the previous year or who may be at risk for one. There is not enough evidence that the strategy, called doxy-PEP, works for other groups, the agency concluded.

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The new study took a different tack, looking instead at preventing STIs with a daily dose of the drug, which may be better for individuals with frequent exposure to infections.

Some providers have suggested that patients might balk at taking antibiotics every day. But many patients told the researchers it would be easier for them to remember to take a daily dose along with a daily pill for HIV prevention or treatment, said Dr. Troy Grennan, who heads the HIV program at the BC Center for Disease Control in Vancouver and led the study.

“Health care providers and researchers make a lot of assumptions about what the community would prefer, and they are often wrong,” he said.

A 2015 study looked at the daily use of doxycycline before sex for preventing STIs in gay and bisexual men who have HIV, but the sample was small, and there was no placebo group.

Inspired By HIV Study

Inspired by that study, Grennan and his team tested the approach in 41 gay and bisexual men in Toronto and Vancouver. The men were already taking a daily pill to treat HIV, and they added 100 milligrams of doxycycline each day for 48 weeks.

The researchers tested the men for STIs every three months and monitored microbial resistance to the antibiotic. They found that daily doxycycline slashed the rate of syphilis infections by 79%, chlamydia by 92% and gonorrhea by 68%.

The team has since found similar results in gay and bisexual men and trans women who do not have HIV.

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Grennan said he was surprised to see that the strategy worked so well against gonorrhea, because about 60% percent of cases in Canada are resistant to tetracyclines, a class of antibiotics that includes doxycycline.

A similar study in France, where the rate of gonorrhea resistance is 65%, found that doxycycline after sex did not prevent new cases. Steady exposure to the antibiotic might be more effective than a single dose that drops off quickly, Klausner said.

Consistent with prior studies, the use of doxycycline to prevent STIs did not seem to increase microbial resistance to the antibiotic. But the study was too small to be sure, some experts warned.

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This article originally appeared in The New York Times.

By Apoorva Mandavilli/National Institute of Allergy and Infectious Diseases via The New York Times
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